1. Technical Field
The present invention relates to a gas sensor element for sensing the concentration of a specific component in a gas to be measured (to be simply referred to as a measurement gas hereinafter) and to a gas sensor that employs the gas sensor element.
2. Description of Related Art
In recent years, from the viewpoint of global environmental protection, the availability of gasoline direct-injection engines and alternative fuel engines, such as CNG (Compressed Natural gas) engines, has been investigated. Accordingly, gas sensors for use in combustion control of the gasoline direct-injection engines and alternative fuel engines have become a focus of attention.
As gas sensor elements to be incorporated in those gas sensors, there are known ones which include: a solid electrolyte body having oxygen ion conductivity and an opposite pair of first and second surfaces; a measurement electrode provided on the first surface of the solid electrolyte body so as to be exposed to a measurement gas; a reference electrode provided on the second surface of the solid electrolyte body so as to be exposed to a reference gas; and a porous diffusion-resistant layer through which the measurement gas is introduced to the measurement electrode.
However, the known gas sensor elements have the following problem when the measurement gas is exhaust gas from an internal combustion engine of a motor vehicle.
Since hydrogen (H2) has a smaller molecular weight than oxygen (O2), the flowing speed of hydrogen contained in the exhaust gas through the diffusion-resistant layer is higher than that of oxygen contained in the same. Consequently, the hydrogen reaches the measurement electrode earlier than the oxygen, so that the partial pressure of oxygen at the measurement electrode becomes lower than the actual partial pressure of oxygen in the exhaust gas. As a result, the output (e.g., the output current or the output voltage) of the gas sensor element is deviated from the correct value that represents the actual concentration of oxygen in the exhaust gas.
In particular, in the case of the engine being a gasoline direct-injection engine, during its operation (including starting operation), the engine tends to generate more hydrogen than a conventional gasoline engine due to the difference in combustion mechanism therebetween. Moreover, in the case of the engine being a CNG engine, during its operation, the engine also tends to generate more hydrogen than a conventional gasoline engine due to the difference in composition between CNG and gasoline. Therefore, in both the cases, the output deviation of the gas sensor element due to the hydrogen contained in the exhaust gas may be significant.
To solve the above problem, there is disclosed a technique in, for example, Japanese Patent Application Publications No. 2007-199046 and No. 2010-276530. According to the technique, a porous catalyst layer is formed on the outer surface of the diffusion-resistant layer; the catalyst layer contains catalytic noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh). Consequently, when the exhaust gas passes through the catalyst layer, part of the hydrogen contained in the exhaust gas will be burnt due to catalysis by the catalytic noble metals. As a result, it is possible to suppress the output deviation of the gas sensor element due to the hydrogen contained in the exhaust gas.
On the other hand, the gas sensors are generally required to have quick responsiveness to change in the concentration of oxygen in the exhaust gas from the engine.
However, the components (e.g., H2, CO, O2) of the exhaust gas will cause oxidation and reduction reactions of the catalytic noble metals contained in the catalyst layer of the gas sensor element. Further, due to the oxidation and reduction reactions of the catalytic noble metals, the concentration of oxygen at the measurement electrode of the gas sensor element will change in proportion to the time and speed of the oxidation and reduction reactions. Consequently, a response delay of the gas sensor will occur due to the change in the concentration of oxygen at the measurement electrode.
The response delay of the gas sensor may be suppressed, in other words, quick responsiveness of the gas sensor may be secured by specifying the percentage contents of Pd and Rh in the catalyst layer of the gas sensor element within predetermined ranges as disclosed in Japanese Patent Application Publications No. 2007-199046 and No. 2010-276530.
However, by specifying the percentage contents of Pd and Rh in the catalyst layer as disclosed in the above two patent documents, it is possible to reliably secure quick responsiveness of the gas sensor only when the exhaust gas is changed from lean to rich. In other words, it may be difficult to reliably secure quick responsiveness of the gas sensor when the exhaust gas is changed from rich to lean only by specifying the percentage contents of Pd and Rh in the catalyst layer as disclosed in the above two patent documents.